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Remembered forever

Rob Coppinger finds that ultrafast lasers could make quartz the data storage medium for eternity

A 100W femtosecond laser could be the means to write data in a quartz storage medium faster and more durably that current technologies, following a successful European research project.

Organisations face the problem that storing information long-term requires copying data from hard disk to hard disk every few years because of the relative short life span of disk technology. The storage of data in quartz, however, will provide a very long-term storage medium – longer than a human life.

Researchers at the University of Southampton’s Optoelectronics Research Centre (ORC), in collaboration with the Netherland’s Eindhoven University of Technology, demonstrated the recording and retrieval processes using a 6W laser operating at a pulse width of 300 femtoseconds and at a one-micron wavelength. But it took three hours to write the 300 kilobits of data, which was a copy of a text file. The file was written in three layers of nanostructured dots, each separated by less than five micrometres.

Such a form of storage could eventually lead to media with a 360-terabit capacity and a thermal stability of up to 1000°C. To reach this goal, the next step is to speed up the writing process with a higher-power, faster laser. The researchers wish to use a 100W laser with a 1MHz repetition rate and a smaller wavelength. The smaller wavelength will allow for a smaller spot size. This will mean the nanostructured spots will be smaller, and so more data can be stored in a given volume of quartz.

The data was written into the quartz in five dimensions. The five dimensions are the x, y and z coordinates within phase retardance, and slow axis orientation of the laser-induced birefringence (which refers to the material having two distinct indices of refraction, associated with different crystallographic directions). The refraction affects the polarisation properties of the material. The laser causes the indices of refraction at a point to alter, to represent a one, instead of a zero, and this change can be detected by combination of optical microscope and a polariser, similar to that found in polaroid sunglasses. The researchers refer to the laser induced change as nanostructuring.

The research is led by Jingyu Zhang, from the university’s ORC. He explained how the data dots are created: ‘We are working on a specific region to generate birefringence. It creates a nano grating, which forms according to the polarisation of the beam. It introduces birefringence which has slow axis orientation.’ Multiple dots can also be created with one laser exposure and with more powerful lasers Zhang expects, ‘more modification [dots] in a single exposure’.

The work was done in the framework of the multi-million euro European Union project Femtoprint. This project is to develop a printer for microsystems with nano-scale features fabricated out of glass.

As well as Zhang’s five dimensional data recordings in glass, this low-power femtosecond laser process could be used to form integrated optics components or create three-dimensional structures like fluidic channels and micro-mechanical components, with the help of chemical etching. Zhang and his colleagues are now looking for industry partners to commercialise this new technology. Zhang says: ‘We are developing a very stable and safe form of portable memory using glass, which could be highly useful for organisations with big archives. At the moment, companies have to back up their archives every five to 10 years because hard-drive memory has a relatively short lifespan.’